Lipopolysaccharide antigens are important indicators of the presence of certain pathogenic bacteria, notably Chlamydia. Such assays require the preparation of an aqueous extract from a clinical sample suspected of containing the organisms. The sample may, for example, be in the form of a clinical swab taken from a patient. An extraction buffer is used to free lipopolysaccharide antigen from organisms that may be present in the sample. A typical extraction buffer will contain lysing agents such as detergents. The extraction procedure may involve heating of the sample in the extraction buffer, for example to a temperature in the range of 80.degree. to 100.degree. C. After several minutes a sufficient quantity of lipopolysaccharide antigen will have been released from any organisms present to provide basis for an accurate assay.
It is already well known that clinical samples often contain irrelevant components which may interfere with the sensitivity of a lipopolysaccharide antigen assay. Cations are a particular nuisance. EP-A-392865 discusses this problem and recommends that divalent cations should be removed from the system, for example, by chelation with agents such as EDTA. EP-A-392865 is particularly concerned with the unwanted presence of divalent cations such as zinc and magnesium. Other cationically-charged materials may also be present in the clinical sample. Any of these cationic materials may interfere with the sensitivity of a lipopolysaccharide antigen assay. We believe that this interference arises because the cations bind to a negatively charged region of the lipopolysaccharide molecule and block an important binding site against which antibodies useful in such an assay are raised.